Resistor Limiting Element Voltage

Discussion in 'General Electronics Chat' started by mcgyvr, May 9, 2012.

  1. mcgyvr

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    Oct 15, 2009
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    Can someone explain what the "Limiting Element Voltage" (LEV) is for a resistor is? and how you determine if your circuit is exceeding that rating?

    I was picking out a resistor to use in a circuit I have and came across this rating on the datasheet. I was specifically looking at a 0402 SMT resistor with a rating of 50V for the limiting element voltage.

    Specifically in the attached circuit. As Vin could be up to 60V are R1 and R3 resistors being exposed to a voltage above their 50V rating? and as such I would need to find a resistor with a higher LEV voltage ????


    OR (for example with R3) is it simply using ohms law V=IR where I=.01A and R=169 so V=1.69 which is less than 50V so its ok?
     
  2. #12

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    R1 is in danger.

    as for the rest, yes. Limiting Element Voltage is the highest voltage safe for that part.
     
  3. mcgyvr

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    and is that because of OHMS law..Where R1 is doing 4mA(0.004A) * 15K = 60V (more than 50V)
    But R3 is okay because .01A x 169 ohms = 1.69V (less than 50V) ?????

    (except R1 is actually 1/2W which is a 1210 SMT package and has a 200V LEV rating)
     
    Last edited: May 9, 2012
  4. crutschow

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    Yes, R3 is fine.

    Your simulation circuit is rather unusual in that you have the pulse source floating above ground. Is that true in the actual circuit?
     
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  5. mcgyvr

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    SGT Wookie did that circuit for me a while back.. He said that for some reason LTSpice needed that R4 resistor there..but in reality its shouldn't be.. But I have removed it and put the source right to ground and didn't notice any real difference in the functionality of the circuit.
     
  6. #12

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    Thanks for covering this, crutschow. I had to go to work today and couldn't get back to clarify followup questions.
     
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  7. mcgyvr

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    yes thanks to both of you.
     
  8. Ron H

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    What is the circuit supposed to do? It doesn't make sense to me.
     
  9. #12

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    A 60 volt pulse is limited by a constant current circuit so that it safely fires an optocoupler with a 10k pull up resistor on its open collector output.
     
  10. Ron H

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    The 9Meg resistor threw me. It would have 120kV across it, as would the optocoupler. I reread post #5, and understand that part now.
    Apparently the input will (or can) go all the way to 60V, but the output needs to switch when the input is only a few volts. The current limiter protects the opto input, while consuming less power than a shunt limiter.
    Is that the gist of it?

    BTW, if the voltage across R1 is a problem, use two 7.5k resistors in series.
     
  11. mcgyvr

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    yes I need to pull a micro pin low when the input voltage is in the range of 5-60VDC

    R1 really isn't a problem because its 1/2W which puts it into a 1210 SMT package which has a LEV of 200V versus a 0402 package with LEV of 50V
     
  12. Ron H

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    I don't know how much you are willing to pay for "better" performance, but check this out.
     
    Last edited: May 10, 2012
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